loggo: lots of minor changes

loggo: lots of minor changes

I've made quite a few changes here, mostly to make
things more Go idiomatic, to add documentation,
and hopefully to make things a bit cleaner and
in one or two cases a little more efficient.

I've added a final "f" onto all the Error, Warning, etc
functions so that they are amenable to checking with
govet. I've also taken out the special casing for no
arguments - if the first argument is a printf-like
format string, it should work as one regardless of
whether there are arguments or not.
If we want a more efficient version, we could add
Logger.Error(msg string) function or similar.

There seemed to be a general mixing of the term
"logger" and "module" so I have changed things
to standardise on "Logger" (as that's the type name)
and changed the Module method and some documentation
accordingly.

I've also made renamed ConfigureLogging to ConfigureLoggers
and DumpLoggingLevels to LoggerInfo and made ConfigureLoggers
understand the format produced by LoggerInfo, which seems
to me a fairly obvious thing to do. I've also made ConfigureLoggers
more tolerant of white space. I haven't in this branch, but in
a subsequent branch I am tempted to propose case insensitivity
throughout - I think that's actually a benefit, as well as
being more efficient. I think it would be best to standardise
on a single separator too - probably semicolon.

To go along with usual Go standards, I've also made functions
that parse stuff return an error if the parsing fails. I think it's
useful to be able to know if ConfigureLoggers is given an
invalid specification, for example.

I've changed the default formatting to print the time in UTC - if
we're not printing the time zone, I think that's important.
I'd quite like the default formatter to print millisecond resolution,
but that can wait.

Things I've thought of but have not done:
- We could make Logger a pointer type and lose the module type. It's
reasonable to say that a nil logger is equivalent to the root logger,
I think and get the same ok-to-use zero Logger semantics.  YMMV about
that, so I left it for discussion.

- We could make ReplaceDefaultWriter more general, say ReplaceWriter,
because the functionality it provides (atomically replacing a writer)
is not possible to get otherwise - it's more than a convenience method.

Please feel free to say which things you don't like and I'll revert
those changes. In general I like the package quite a bit - thanks for doing it!

https://code.launchpad.net/~rogpeppe/loggo/minor-fixes/+merge/167832

(do not edit description out of merge proposal)

[rog, 2013-06-06 19:14:29]

Please take a look.